S. marcescens on an agar plate.
| Serratia marcescens|
Description and significance
Serratia marcescens are Gram-negative bacteria which fall under the tribe Klebsielleae and the large family Enterobacteriaceae. It is a motile bacterium with flagella. It is also a facultative anaerobe. The species, Serratia marcescens is the main pathogen under the genus Serratia. Strains of S. marcescens produce prodigiosin, the pigment that gives the bacteria its unique red color. They are rod shaped bacillus bacteria. It is found in numerous different environments. As a human pathogen, however, it is primarily contracted by hospital patients resulting in urinary and respiratory tract infections. It is also resistant to numerous antibiotics.
Cell Features and Functions
Serratia marcescens are facultative anaerobes, which means they prefer oxygen as a source but can do with out. The red pigment characteristic to the bacteria is produced by the condensation of the enzyme prodigiosin. S. marcescens have other distinct features. As opposed to other Gram-negative bacteria, they can perform casein hydrolysis; producing metalloproteinases which allow cell-to-extracellular matrix interaction. For different metabolic processes to function S. marcescens also degrade tryptophan and citrate. Citrate is a carbon source for S. marcescens. Researchers have discovered other features of the bacteria, such as adherence and hydrophobicity, and lipopolysaccharides (LPS) that contribute to its pathogenic nature. In a methyl red test, the bacteria tests negative because it does not perform mixed-acid fermentation. It also produces lactic acid resulting from oxidation and fermentation. Different enzymes have been identified in S. marcescens that contribute to its pathogenic nature, such as chitinase, lipase, chloroperoxidase and an extracellular protein, HasA .
The Sanger Institute was funded by the Wellcome Trust, and CNRS, to sequence the genome of Serratia marcescens strain Db11. Dr. Jonathan Ewbank of the Centre d'Immunologie de Marseille Luminy is involved in the sequencing as well. . The genome was completed and consists of a single circular chromosome of 5,113,802 base pairs with a G+C content of 59.51%.(1040779 A; 1522992 C; 1520315 G; 1029716 T; 0 other) The shotgun reads are still available. The database contains 80,227 reads totaling 51.619 Mb and giving a theoretical coverage of 99.99% of the genome .
Ecology and Habitat
S. marcescens can be found in various places. They are mesophilic bacteria that prefer moderate or preferably damp environments. Their optimal temperature is at 38C. Extremely low or high temperature and even pH are lethal to S. marcescens. It can be found in soil, or water, or in an infected host. Ranging from vertebrates, invertebrates, including plants, S. marcescens can infect almost anything. In humans they are mainly found in the urinary and respiratory tract, but any part of the body is susceptible. 
S. marcescens was discovered in 1819 by Bartolomeo Bizio, an Italian pharmacist from Padua. He named the bacteria after the Italian physicist Giacinto Serrati. He first spotted it on cornmeal mush, polenta. The red color on the mush was S. marcescens. For along time the bacterium was not seen as a harmful pathogen but rather a harmless bacterium to experiment with. In the late 1800’s it became known as an infectious pathogen.
S. marcescens is a common cause of infections in hospitals, or nosocomial infections. Transmission could be as simple as shaking a hand. In hospitals they are especially existent in the neonatal unit, or ICU. Also contaminated hospital equipment cause transmission. Syringes, catheters, solutions and other things may have the bacteria on them and cause transmission. That is why it can be observed in heroin addicts who use syringes. Treatment includes cephalosporins, gentamicin, amikacin. However, resistance to the antibiotic is an innate characteristic, formed from r-factors on the plasmids of the S. marcescens. The bacteria have a resistance to ampicillin, macrolides, and first-generation cephalosporins. Antibiotics used to treat Serratia infection include beta-lactam agents, aminoglycosides and fluoroquinolones and a variety of different resistance mechanisms have been demonstrated It can cause infection anywhere even in the eye causing conjunctivitis, or keratitis..
Application to Biotechnology
Since the early 1900s, the bacteria were widely used in biotechnology because it was believed to be non-pathogenic. It was used in experiments to track infection, easily done because of the red pigmentation. It was even used in school experiments. In fact, teachers and scientists commonly used S. marcescens in experiments of microbial transmission and to demonstrate the importance of hand washing. Until the 1950s, S marcescens was considered a harmless saprophyte. S marcescens was then recognized as an opportunistic pathogen in humans, and other organisms. It is still used as a biological marker for the transmission of microbes. In Drosophila research, S. marcescens is a commonly used bacterium to cause infections. The S. marcescens can be observed by its distinct color, or plaques. In respects to epidemiological studies, S. marcescens can be detected using biotyping, bacteriocin typing, phage typing, plasmid analysis, polymerase chain reaction and ribotyping. RAPD- polymerase chain reaction is effective in rapidly monitoring an outbreak and tracing the source of initial infection.
"Antagonism of Serratia marcescens towards Phytophthora parasitica and its effects in promoting the growth of citrus"
Phytophthora parasitica causes serious widespread disease in citrus called gummosis. The disease exhibits symptoms of rotting of roots. A strain of Serratia marcescens R-35, isolated from citrus (a hybrid between mandarin orange and lemon), suppressed more than 50% of the disease and promoted the growth of the citrus plant. This study exhibited biological defense rather than pesticides as a way of eliminating the disease. The experiment was done under greenhouse conditions .
"Kinetic analysis of growth rate, ATP, and pigmentation suggests an energy-spilling function for the pigment prodigiosin of Serratia marcescens"
This research experiment used a kinetic model relating cell, ATP, and prodigiosin concentration changes for S. marcescens during cultivation in batch culture. Cells were grown in a variety of complex broth media at temperatures which either promoted or essentially prevented pigmentation. High growth rates were accompanied by large decreases in cellular prodigiosin concentration; low growth rates were associated with rapid pigmentation. Prodigiosin was induced most strongly during limited growth as the population transitioned to stationary phase, suggesting a negative effect of this pigment on biomass production. Mathematically, the combined rate of formation of biomass and bioenergy (as ATP) was shown to be equivalent to the rate of prodigiosin production. Studies with cyanide inhibition of both oxidative phosphorylation and pigment production indicated that rates of biomass and net ATP synthesis were actually higher in the presence of cyanide, further suggesting a negative regulatory role for prodigiosin in cell and energy production under aerobic growth conditions. Considered in the context of the literature, these results suggest that prodigiosin reduces ATP production by a process termed energy spilling. This process may protect the cell by limiting production of reactive oxygen compounds. Other possible functions for prodigiosin as a mediator of cell death at population stationary phase are discussed.
"Outbreak of Serratia marcescens in a neonatal intensive care unit: contaminated unmedicated liquid soap and risk factors"
This research study is about a Serratia marcescens in a neonatal intensive care unit (NICU). It also dealt with the control measures taken in the NICU. The study was took place in a three-month period, in which five infants were infected S. marcescens. Results of culture surveys and pulse-field gel electrophoresis indicated a soap dispenser bottle was to be blame for the spread of the infection. The reservoir of S. marcescens in the soap dispenser was not the only source, but infants with S. marcescens infection were possibly exposed to a venous catheter. The study showed that though soap dispensers are meant to keep good hygiene, they can also be the source of nosocomial infections. Control measures established stated that using airless soap dispensers would be ideal to keep S. marcescens out. Also to control outbreak hand sanitizer should also be used. These measures led to the successful control of the S. marcescens outbreak. 
- http://emedicine.medscape.com/article/228495-overview Basilio J Anía, MD “Serratia” eMedicine. 1 Oct 2009
- http://jmm.sgmjournals.org/cgi/content/abstract/46/11/903.[Hejazi, A. ; F. R. Falkiner1 “Serratia marcescens” J Med Microbiology 46 (1997), 903-912; DOI: 10.1099/00222615-46-11-903. April 1, 1997.
- PMID: 9368530 [PubMed - indexed for MEDLINE] http://www.ncbi.nlm.nih.gov/pubmed/9368530
- http://microbezoo.commtechlab.msu.edu/zoo/microbes/serratia.html. "The Miracle Microbe: Serratia marcescens." Digital Learning Center for Microbial Ecology. Michigan State University, National Science Foundation, 1999.
- http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1517-83822006000400008&tlng=en&lng=en&nrm=iso “Antagonism of Serratia marcescens towards Phytophthora parasitica and its effects in promoting the growth of citrus” Brazilian Journal of .Microbiology 37(4). São Paulo. Dec. 2006.
- http://www.ncbi.nlm.nih.gov/pubmed/18805986. Jones, S, et al. "Kinetic analysis of growth rate, ATP, and pigmentation suggests an energy-spilling function for the pigment prodigiosin of Serratia marcescens." Journal of Bacteriology 190(22): 7453-63. Epub 2008 Sep 19.
- http://www.ncbi.nlm.nih.gov/pubmed/19246120 “Outbreak of Serratia marcescens in a neonatal intensive care unit: contaminated unmedicated liquid soap and risk factors.” Journal of Hospital Infection 72(1):17-22. Epub 2009 Feb 25.